The polymers of vinyl halide monomer, particularly vinyl chloride, constitute an important group of thermoplastics. Such polymers have been prepared by various techniques including suspension and emulsion mode polymerization. However, the latter modes of polymerization which employ aqueous reaction conditions are meeting increasing competition from the bulk mode of polymerizing vinyl halide which omits the inconvenient aqueous diluent.
A serious problem has recently arisen in product of polyvinyl halide in connection with the discovery that vinyl halide monomers are potential or actual health hazards to humans.
For example in the case of vinyl chloride monomer, recent experiments on animals have indicated that the monomer is carcinogenic. Accordingly, the Federal Government has determined that action should be taken to reduce human exposure to the monomer. Such exposure is a particular serious problem to workers engaged in manufacture of polyvinyl chloride. This is so since the product of the polymerization is a vinyl chloride polymer containing minor amounts, e.g. 100 to 500 parts per million (ppm) of the monomer in a residual free form. The polymer product is obtained as a finely divided resin and the residual monomer will slowly diffuse from the resin as a gas. Such natural diffusion, however, normally requires many days for the monomer content of the product polymer to reach a relatively low level, e.g., less than 50 ppm. During this time, while newly-made polymer is undergoing any manufacturing operations, subsequent to polymerization, workers are exposed to the residual unreacted monomer from the polymer which escapes into the surrounding atmosphere.
To avoid the aforementioned health hazard, there have been devised processes for treating polymerized vinyl halide polymers to remove substantially all of the vinyl halide monomer residue from the polymer.
One such process is described in F. A. Cich U.S. Pat. No. 4,205,149, issued May 27, 1980. According to patentees' process, bulk liquid phase polymerization of vinyl chloride is carried out in the presence of a diluent for the reaction, inter alia an alkane hydrocarbon, e.g. methane, propane, butane, i.e. n-butane, and higher alkanes of straight, branched, or cyclic structure. Upon release of the superatmospheric pressure under which the bulk liquid phase polymerization is carried out, a substantial part of the diluent and unreacted monomer is immediately removed by evaporation from the crude polymer containing reaction mass. The residues of unreacted monomer and diluent in the crude polymer are then removed by a degassing step in which the reaction product is subjected to diminished pressure, e.g. from about 300 mm to about 0 mm, optionally while heating the crude polymer at a moderately elevated temperature, which is below the temperature at which the vinyl halide polymer begins to decompose, e.g. at about 50.degree. to about 85.degree. C.
The foregoing degassing process effectively removes the hazardous residues of the monomer from the polymer to provide a product which contains only about 31 ppm of vinyl halide monomer when the short chain hydrocarbon alkane n-butane is used as diluent in patentees' bulk polymerization process (see Table II, Example 13 of the patent).
It has been found, however, that the aforementioned vinyl halide polymer prepared by bulk liquid phase polymerization in the presence of n-butane according to U.S. Pat. No. 4,205,149 is characterized by relatively poor color stability during the period immediately subsequent to melting. Moreover, while patentees' polymer is characterized by an improved porosity (with respect to the polymer's ability to absorb liquid polyvinyl halide additives, e.g. organic plasticizers) compared to polymer manufactured without the diluent, a further improvement in the aforementioned porosity property would be highly desirable.